Communication signals transmitted across a communication medium propagate at a finite speed. Hence, the signals encounter delay between the time of their transmission from a source node and the time of their reception by a destination node.
It is often important to know how long the delay is. For example, in synchronous communication systems the delay must be known so that clocks at the source and destination nodes which control the transmission and reception of communication signals, respectively, may be synchronized with each other. And in voice communication systems, where propagation delays degrade the capability of carrying on spontaneous, face-to-face like, conversations and introduce other undesirable effects such as echoes, the delay must be known to determine whether satisfactory communication between nodes is feasible.
In communication systems where signal transmissions from a given source node to a given destination node follow a predetermined and known path, calculation of the transit delay is often a simple matter, even before any signal has been transmitted across the path. The transit delay calculation involves multiplication of the signal propagation characteristic of the transmission medium connecting the source and destination nodes by the distance between those two nodes.
However, in many communication systems this technique cannot be employed, because either the propagation characteristic of the medium or, as is more likely, the distance between the source and destination nodes is not known. In telemetry systems, for example, where the source and destination nodes may be a satellite moving over the surface of the earth and a tracking earth-based antenna, the distance between the source and destination points constantly varies. And in switching networks, such as packet switching networks, for example, there are a number of paths of different lengths that connect the source and destination nodes, and a packetized message may follow any one of these paths; thus, unless a message carries with it information identifying the path it has followed, its transit delay is not easily calculable by the above-described method.
Furthermore, in a switching network such as the packet switching network, a message may be temporarily stored, and hence delayed, at intermediate nodes in the path that it is traversing, thus further increasing the difficulty of determining the delay it has undergone. In this case, the delay cannot be determined from a knowledge of the traversed path alone.
A technique for measuring that portion of the delay time which the message spends at intermediate nodes is disclosed in U.S. patent application Ser. No. 392,377 to W. A. Montgomery, entitled "Time Stamping for a Packet Switching System", filed June 25, 1982, and assigned to the same assignee as this application. The technique involves including a time stamp, of a value known to both the source and destination nodes, in every message sent through the network. The time stamp is updated at every node of the network traversed by the message: when a message enters a node, a relative time stamp is applied, and before the message leaves the node, the relative time is used to find the time spent at the node, which time is added to the original time stamp. At the destination node, the updated time stamp is read and the delay time spent at intermediate nodes is determined therefrom.
This technique unfortunately requires the presence of time-indicating mechanisms--clocks--at each node of the network. It also requires synchronization at each node of the time-indicating mechanism that keeps track of when a message enters the node with the time-indicating mechanism that keeps track of when a message leaves the node. Each node likewise requires a mechanism for computing the difference between these two time-indicating mechanisms and for adding this difference to the original time stamp to update it. These requirements increase the complexity and cost of the nodes, and hence of the network. And furthermore, this technique does not account for the transmission delays accumulated by the message in passing between the various nodes.
One solution in prior art systems has attempted to resolve this problem by including in each packet a time stamp indicating when the packet was transmitted from the originating node. When the packet arrived at the destination node, the delay encountered by the packet was determined by comparing the time in the time stamp with the present time. The problem encountered with this solution was that all of the possible originating and destination nodes had to maintain synchronized clocks. The prior art solution was not a practical solution, since there existed too many technical difficulties in maintaining synchronized clocks over a large number of nodes. And moreover, synchronization and maintenance of synchronization requires that the delay between the nodes already be known.
In view of the foregoing, there exists a need for improved techniques and facilities for determining the delay encountered by a signal as it traverses a transmission path from its origin to its destination.